The structure according to the invention is similar to the Y-junction device diagrammatically shown in FIG. 1. Such a device comprises an optical input waveguide 10 of direction De, extended by two output guides 11 and 12 of directions Ds.sub.1, Ds.sub.2 which are respectively inclined by +.alpha. and -.alpha. with respect to the input guide direction De. The structure also comprises a widened zone 13 forming the junction between the input guide 10 and the output guides 11, 12. An optical beam entering the junction 13 by the input guide 10 is subdivided into two beams, which propagate into the two output guides 11, 12.
In order to better define the phenomena encountered in such a structure, consideration will be given hereinafter to two planes A and B perpendicular to the direction De, said planes defining a zone I to the left of A and which is that of the input guide 10, a zone II between A and B, which is that of the actual junction 13 and a zone IIII to the right of B, which is that of the output guides 11, 12. Moreover, C is the edge at which the two output guides 11, 12 meet. Such a Y-junction device has numerous applications in integrated optics, particularly in so-called Mach-Zender modulator constructions.
The properties of these devices are, for example, described in the article by I. ANDERSON, entitled "Transmission performance of Y-junctions in planar dielectric waveguides", published in the journal "IEE Proceedings of Microwaves Optics and Acoustics", 2, pp. 7 to 12, January 1978, as well as in the article by BAETS R and LAGASSE P. E., entitled "Calculation of radiation loss in integrated optics tapers and Y-junctions", published in the journal "Applied Optics" 21, 11, pp. 1972-1978 of June 1982.
This type of device suffers from three difficulties, which must be understood in order to appreciate the interest of the present invention. These difficulties are:
(a) the mode conversion in the junction region (region II), PA1 (b) the appearance of radiation in the junction angle (zone C), and PA1 (c) coupling between the two output guides (region III).
The mode conversion phenomenon is as follows. The input guide 10 and the output guides 11 and 12 are designed for monomode operation. This means that, for the operating waveguide length used, the width and thickness of the guides are such that only the fundamental propagation mode can be established. For a given thickness, the tolerance on the guide width is very small. If this width is excessive, higher order modes will be able to propagate. This is precisely what takes place in zone II of the device, where junction 13 has a widened shape, whose width increases constantly between plane A and plane B. Thus, the condition of maintaining the lowest mode is not respected in this zone, and higher order modes can appear. The diffraction of light in plane A is combined with the aforementioned phenomenon to increase this mode conversion. Thus, in this plane, the guide has a break. In plane A, the propagation vector along De in zone I has an angular dispersion. Accordingly, following A, the propagation vector will no longer be directed along De in the whole cross-sectional plane, the vector being obliquely directed on the edges.
These two phenomena combine to break the monomode character of the structure and bring about the mode conversion. As, by design, output guides 11 and 12 are monomodal, there will be incompatibility between the multimode wave front which reaches these guides in plane B and the monomode wave front able to propagate in the two output guides. Thus, part of the input light energy will be dispersed in the region of plane B.
The second difficulty encountered in junctions of this type is the appearance of radiation in angle C. The quasi-spherical front of the wave propagating from plane A to plane B in region II strikes edge C, where a diffraction wave is formed, whose centre is the said edge. This wave radiates throughout the device, including towards the waveguide. Only a small part of this diffracted wave satisfies the conditions permitting the propagation in the output guides, the remainder being diffused and lost.
The final difficulty concerns the proximity of the output guides in region III, which has the effect of coupling the guides to one another causing (as in the directional coupler where this effect is used) the transfer of energy from one guide to the other. This transfer has a quasi-periodicity on moving away from plane B. The smaller the value of the angular aperture (2.alpha.), the more marked this effect. However, in the Y-junctions, this angle is necessarily small (only a few degrees) if it is wished to obtain a good energy transfer from the input guide to the output guides, so that said coupling is important.